Electrodeposition of chromium



Nov. 20, 1956 R. s. DEAN ETAL 2,771,413

ELECTRODEPOSITION OF CHROMIUM Filed June 2'7, 195]. 2 Sheets-Sheet l Chromite ore smelted with carbon to produce ferrochrome Ferrochrome slowly cooled, broken up and comminuted Leach with solution of (NH S0 H 80 Cr$0 y 7 Filter A Undissolved ferrochrome Filtrate Dissolve in excess H 80 (NH SO l Fil ter Residue filtiate Add excess comminuted ferrochrome l Fil ter Ferrochrome Fi ltrate l Heat to +80-lO0"C Goal to 5C to crystallize FeSOJNHQSO l Filter A l v FeSO (NH SO crystals Mother+liquor Heat to 80-l00C l Cool and seed with Cr alum l Filter Cr alum crystals Mother liquor l l Recrystallize Dissolve in water l Add (N ml so l Feed to cathol'yte compartment of cell for electrodeposlting Chl'OmlUln INVENTORS Regina/d 5. Dean E91 y Ho/berf E Dunn THE/R ATTORNEYS Nov. 20, 1956 R. s. DEAN ElAL 2,771,413

ELECTRODEPOSITION OF CHROMIUM Filed June 27, 1951 2 Sheets-Sheet '2 Chromite concentrate smelled with carbon to produce ferrochrome Ferrochrome cast into anodes Anodic solution of ferrochrome anodes in anolyte containing H 80 (NH S0 Remove anolyte from cell and add comminuted ferrochrome b Heat solution to 90C Cool to 0C Filtler Fe$0 (NH SO crystals Filtrate Allow to stand to crystallize Cr alum l Fil ter Cr alum crystals Motner liquor l L Dissolve in water A (N ;l s0

Clarify l. Feed to catholyte compartment of cell for electrodepositing chromium IN VEN TORS Regina/d 5. Dean y Ho/berl E Dunn 144,44 MYANM- THF/R ATTORNEYS ELECTRODEPOSITION F CHROMIUM Reginald S. Dean, Washington, D. C., and Holbert E. Dunn, Crafton, Pa.

Application June 27, 1951, Serial No. 233,786

3 Claims. (Cl. 204-105) This invention relates to the production of electrolytic chromium and particularly to methods for producing electrolytic chromium from chromium ores and chromium concentrates.

In the known art electrolytic chromium has been produced from chrome ores by dissolving such ores in sulphuric and chromic acid, for example in accordance with U. S. Patent No. 2,507,476, granted May 9, 1950, to Rex R. Lloyd. By such a procedure all of the constituents of chrome ores, including in addition to chromium the impurities iron, aluminum and magnesium, are dissolved in the solution as sulphates. The purification of such a solution, for example, by fractional crystallization, has proved a difiicult and expensive procedure.

We have found that the production of electrolytic chromium can be greatly facilitated by a new combination of steps, the principal steps being as follows.

Step 1.The chromite ore is smelted with carbon to form ferrochrome and a slag containing most of the other ingredients of the ore such as magnesium, aluminum and silica.

Step 2.-The ferrochrome is separated from the slag and brought into suitable form for further treatment.

Step 3.The ferrochrome is dissolved in a solution containing sulphuric acid and ammonium sulphate under such conditions as to form a solution containing ferrous iron and chromic and chromous salts. Step. v4..The solution is purifiedto remove iron.

Step 5.The purified chromium solution is electrolyzed to produce electrolytic chromium and regenerate a solution of sulphuric acid and ammonium sulphate for use in Step 3.

In the accompanying drawings, Figures 1 and 2 are flow sheets of Examples 1 and 2 respectively, hereinafter described in detail. r t

In carrying out Step 1 of our process wherein chromite ore is smelted with carbon .to form ferrochrome, we prefer to use an electric furnace .and to carry out'the 'smelting in such manner as to produce ferrochrome containing 50-75% chromium, 2030%' iron and 4-9% carbon. This can be accomplished in the following manner.

The chrome ore to be reduced to ferrochromium is mixed with carbonaceous reducing agent and flux, selected with respect .to sizing, so .thatundue pack-ing ofthe charge in the furnace crucible is avoided, While providingsufficient porosity of the charge for free escape of the gaseous products of the operation; viz., carbonrnonoxide, entrained free moisture and combined water as well. The reducing agent is proportioned to provide sufiicient fixed carbon to'theoretically reduce all of the chrome oxide to metallic chromium and all of the iron oxide content to metallic iron, as well as sufiicient to .reduce the desired amount of silicon into the ferrochromium product, from thesilica content of the ore and/ or from the silica stone usually required as flux if the natural slag is too viscous,

and in addition suflicient reducer to provide enough carbon to carburizethe ferrochromium product to the desired carbon content 1plus a practical excess, usually ex- I United States Patent Patented Nov. 20, 1956 "ice pressed as percent excess of theory, amounting to 5, 10, or even 20 percent, depending on the running characteristics of the operation for a given ore combination and desired end-product. The theoretical reactions involved are as follows: t

(l) FeO+C=Fe+CO, 1 part FeO requires 0.167 part C by weight. v

(2) CrzO3+3C=2CR+3CO, 1 part CrzOz requires 0.237 part C by weight. i

(3) SiOz+2C=Si+2CO, 1 part SiOz requires 0.40 part C by weight.

Fluxing of the natural slag may be acid, basic, or neutral, as exemplified by silica stone, lime, or bauxite, depending on the silicon and carbon grades of alloy desired.

The reduction may be carried out in a carbon-lined crucible. The normal tapping temperature range is 2800 to 3250 F., usually performed at intervals of 1 /2 to 2 hours, depending on the size of the furnace.

The manner of carrying out Step 2 depends on the procedure adopted for Step 3. There are several possible ways of carrying out the dissolving Step 3 within the scope of our invention. These are designated Step 3A, Step 3B and Step 3C. We prefer either chemical solution or anodic solution of the ferrochrome.

STEP 3A (CHEMICAL SOLUTION OF FERROCHROME) We have found that if the ferrochrome is cooled slowly from its molten condition, it is readily soluble in acid of moderate concentration. Consequently, in dissolving the ferrochrome by chemical solution we use a slowly cooled ferrochrome. We have found that comminuting the ferrochrome by casting it into water or into thin masses which can be easily broken up does not give the desired results. The preferred method of producing the slowly cooled ferrochrome is to cast it in large ingots or reguliand break these up mechanically. Preferably the ferrochrome is cast inreguli about 12 to 14 inches thick. Such reguli require 8-12 hours for cooling. We have found that the cooling time should be at least several hours where the ferrochrome is to be dissolved by chemical solution. Smaller masses can be cooled at the required rate by suitable means of preventing heat loss.

The slowly cooled ferrochrome reguli are broken up and comminuted preferably to about inoh size. The comminuted ferrochrome is dissolved in a solution containing sulphuric acid and ammonium sulphate and usually some chromium sulphate. This solution may be made up of the mother liquor from the crystallization of chrome alum carried out in Step 4, anolyte from Step 5 i and make-up sulphuric acid. The'i-nitial strength ofacid in this solution before addition of make-up acid is usu ally about 20% sulphuric acid. The dissolving of the ferrochrome in this solution is preferably carried on at about 80 C;, although this temperature is not critical.

In order to expedite dissolving of the ferrochrome and at the same time obtain as the end product a solution of chromium, iron, and ammonium sulphates having a pH of about 1-3, we prefer to use a countercurrent form of leaching. That is, a considerable excess of ferrochrome principally of slag inclusions and carbon.

is removed by filtration. This Method 3A produces a. solution of chromium and iron sulphates in which the. chromium is all in the trivalent or bivalent state and the iion'is all 'in' the bivalent or ferrous condition. This con} dition is necessary for effectively carrying out the further steps of our process.

STEP 3B In this embodiment of our invention we obtain the same type of solution by the anodic solution of the ferrochrome. In this embodiment we do not (in Step 2) comminute the ferrochrome nor do we cool it slowly from the melt. On the other hand, we cast it into thin slabs which cool relatively rapidly. We make such slabs an anode in the same kind of spent electrolyte as was used according to Step 3A for dissolving the comminuted ferrochrome, that is, a solution of sulphuric acid and ammonium sulphate containing small amounts of chromium not removed in the other steps of the process.

In order to produce a solution containing the chromium in the trivalent and bivalent form and the iron entirely in the bivalent form, it is necessary to carry out the anodic solution of the ferrochrome in a certain definite manner. To obtain good current efficiency in anodic solution of the fer'rochrome it is necessary to use a compartment cell, that is, an electrolytic cell which is separated into two compartments by an ion-permeable diaphragm but one which prevents mixing of anolyte and catholyte. The solution and electrode of the catholyte are not critical for the anodic solution of the ferrochrome in an anolyte containing sulphuric acid and ammonium sulphate.

The catholyte may advantageously be either purified chromium solution or purified iron solution and the cathode any metal unattacked by such solution. Iron or chromium may accordingly be deposited on the cathode without additional power cost over that required for the anodic solution. From the standpoint of anodic solution of the ferrochrome, however, the catholyte may be any electrolyte, e. g., sulphuric acid, and the cathodic product may be merely hydrogen.

If anodic solution of the ferrochrome is carried out without the addition of a reducing agent, it must be done at a low current density corresponding to an anode potential drop of less than 1.0 volt in order to maintain the iron in the ferrous state, which is necessary for carrying out Step 4 in a satisfactory manner. To permit the use of higher current densities, we prefer to add sulphur dioxide to the anolyte. This not only maintains the iron in the ferrous state but lowers the anodic voltage drop for a given current density, that is, it decreases passivity. In addition, the oxidation of the sulphur dioxide by ferric iron formed furnishes additional sulphuric acid which would otherwise have to be added in regenerating the electrolyte. The amount of sulphur dioxide added preferably is just enough to maintain the iron in the ferrous state.

STEP 30 An alternative procedure to anodic solution of the ferrochrome in a two-compartment diaphragm cell is the electrolytic solution in a single compartment cell in which an alternating current is passed through the electrolyte of sulphruic acid and ammonium sulphate using two ferrochrome electrodes. In this way all the iron is maintained in the ferrous condition and the chromium is essentially trivalent.

The solution which is obtained by any of the preferred embodiments of Step 3 of our invention now requires to be purified. The preferred method is to heat the solution to EEO-100 C. to stabilize the chromium in the green chromic modification, then cool to a low temperature, as for example 15 C., and crystallize a ferrous ammonium sulphate product containing a small amount of chromium ammonium sulphate, which is removed from the mother liquor. The ferrous ammonium sulphate product may be once recrystallized and discarded or used to produce electrolytic iron in the cathode compartment of a two-compartment diaphragm cell as previou's'ly' described.

The mother liquor from the crystallization of the ferrous ammonium sulphate product contains chromium and ammonium sulphates and residual ferrous sulphate. It is allowed to stand for a considerable period, whereupon chrome alum (ammonium chromic sulphate) crystallizes in the pure form, which after separation from the mother liquor is redissolved to provide a suitable electrolyte for depositing electrolytic chromium.- The chrome alum may be recrystallized to give additional purity in the electrolyte for use in Step 5.

The deposition of chromium is carried out in the cathode compartment of a diaphragm cell in which any substantial amount of the anolyte is prevented from diffusing into the catholyte. One preferred diaphragm is made of a plastic cloth such as Vinyon which has been rendered less permeable by coating with rubber or acrylic resin. Other diaphragms may be used, it being essential only that diffusion of the anolyte into the catholyte be prevented with the interposition of minimum electrical resistance. The level of liquid in the catholyte compartment is maintained slightly higher than that in the anolyte compartment so that any diffusion is out of the catholyte. The anolyte compartment of the chromium deposition cell may be filled with a solution of ammonium sulphate which will become increasingly acid as oxygen is evolved from an inert anode such as lead or lead alloy. Alternatively the anode compartment of the chromium deposition cell may be used for solution of the ferrochromc in accordance with Step 38.

Having described the various steps of our process and their interrelation, we will now illustrate our invention by certain examples, reference being made to Figure 1 for Example 1 and to Figure 2 for Example 2.

Example 1 (Chemical solution of ferrochrome) We took a chromite ore analyzing- SiOz 6.31

MgO 13.31

CaO 0.15

Fe (as oxides) 11.03

This ore was smelted in an electric furnace with carbon and a flux of silica stone and we obtained ferrochrome ana1yzing 65.50 Cr 22.61 Fe 3.88 Si 7.83 C

This alloy was cast in slabs 14 inches thick and broken up with a sledge and then put through a jaw crusher. It was then comminuted in a hammer mill to pass an 8 mesh screen. One hundred pounds of this comminuted ferrochrome' was treated with 35 gallons of a solution of ammonium sulphate and sulphuric acid containing 0.5 pound per gallon ammonium sulphate, 0.25 pound per gallon sulphuric acid, and 0.12 pound per gallon of chromium as sulphate. The comminuted alloy was allowed to remain in contact with the solution for 24 hours. A substantial portion of the ferrochrome remained undissolved. At the end of this time the undissolved portion of the ferroehrome was separated from the solution and the solution contained chromium in the amount of 0.25 pound per gallon or 30 grams per liter. The solution had a pH of 3.

The undissolved ferrochrome was treated with an excess of a solution containing about 0.5 pound per gallon of sulphuric acid and 0.5 pound per gallon of ammonium sulphate. In 24 hours it was all dissolved except for a small residue consisting of silicaand carbon, which was removed from the solution by filtration. The solution was then treated with van excess of comminuted ferrochrome to produce a solution containing 30 grams per 5 literchromium, 70 grams per liter ammonium sulphate, and having a pH of 3. 'Iheferrochromewhich did not dissolve was filtered off from the solution and returned to an earlier stage of the process where it was treated with a solution of ammonium sulphate and sulphuric acid and chromium sulphate. The solution from which the excess ferrochrome had been separated was heated to 80 l C. to stabilize the chromium in the green chromic modification, cooled to C., at which point a ferrous ammonium sulphate product containing a small amount of chromium ammonium sulphate crystallized and was separated from the solution. The ferrous ammonium sulphate product contained 12.1% iron and 0.6% chromium. The resulting solution contained about 5 grams per liter of iron, about 30 grams per liter of chromium and about 50 grams per liter of ammonium sulphate. The solution was then heated to 90 C.-for 1 hour and then cooled and seeded with chrome alum and allowed to stand 24 hours. The chrome alum crystallized and was separated from the solution. The chrome alum contained 9.5% chromium and 0.7% iron. The mother liquor from which the'chrome alum was removed contained 5 grams per liter of chromium, 5 grams per liter of iron and 20 grams per liter of ammonium sulphate. It was returned to the solution step of the process for dissolving a fresh quantity of ferrochrome.

The ferrous ammonium sulphate product crystals were discarded in this example. However, they may be used to prepare an electrolyte and electrolyzed to yield electrolytic iron as a byproduct of the process. The chrome alum crystals were once recrystallized, which reduced the iron content to 0.1%. The purified chrome alum crystals were dissolved in water to produce a solution having 50 grams per liter of chromium, and 50 grams per liter of ammonium sulphate were added. This solution was placed in the catholyte compartment of an electrolytic cell provided with a diaphragm of Vinyon cloth. The cathode was stainless steel. The anode compartment contained a solution of ammonium sulphate with 70 grams ammonium sulphate per liter. The anode was lead. Conditions of electrolysis were-- Cathode area sq. ft.

Rate of catholyte flow .25 gaL/ sq. ft./min. Current density 70 amps./ sq. ft. Cell potential 4.2 volts.

Current etficiency 65.2%. Kw.-hr./lb. metal 5.16.

Temperature 43 C.

Example 2 (Anodic solution of ferrochrome) A chromite concentrate from Montana analyzing- 41.45 CrzOs (28.35% Cr) 17.50 Fe (as oxides) 16.40 A1203 13.26 MgO 3.68 SiOz was smelted in an electric furnace with carbon and a flux of silica sand to produce ferrochrome analyzing 60.33% Cr 28.86% Fe 3.48% Si 7.33% C This ferrochrome was cast in the form of anodes 1 inch thick and 12 inches wide and 24 inches long. Sometimes ditficulty is experienced in preventing the cracking of such anodes on cooling. It has been found satisfactory to press a strip of lead around the anodes to prevent their falling apart when used. Other forms of anodes such as 1 inch round rods may be cast and a number of such rods connected to form a composite anode. Such rods maybe cast with less difiiculty from cracking. The anodes were set up in the anode compartmentof a cell having a porous diaphragm of fritted alumina. The anolyte contained 75 grams per liter of sulphuric acid and 100 grams per liter of ammonium sulphate. The cathode compartment of the cell had a stainless steel cathode and was filled with a circulating solution of ferrous ammonium sulphate which was maintained at a pH of 6 by adding sulphuric acid. Electrolytic iron was deposited. The anode current density was maintained at 60 amps. per square foot with an anode potential drop of 0.4 volt by adding sulphur dioxide at a rate to keep the iron entirely in the ferrous state. The current efliciency of the anode for dissolving chromium was 57.2% and the amount of sulphur dioxide added was stoichiometrically equivalent to the iron dissolved. The residue remaining after the anodic solution of the ferrochrome was a mixture of silica, carbon and slag particles.

The solution produced by the circulation of anolyte in the anode compartment had the following composition:

100 g./'l. (NH4)2SO4 50 g./l. Cr as sulphate 25 g./l. Fe as sulphate pI I=0.5

This solution was removed from the cell and brought to a pH of3 by adding comminuted ferrochrome thereto. This also serves to reduce any ferric iron present to ferrous iron. The neutralized solution Was heated to 90 C. for 30 minutes and then cooled to 0 C. A ferrous ammonium sulphate product containing a small amount of chromium ammonium sulphate crystallized and was removed from the mother liquor, which contained only 3.5 grams per liter of iron.

The solution from which the ferrous ammonium sulphate product had been separated was allowed to stand for 5 days, at which time about half of the chromium had crystallized as chrome alum, which was separated from the mother liquor. The mother liquor was used for dissolving more ferrochrome. The chrome alum crystals were dissolved in a small amount of water at C. to produce a solution containing grams per liter of chromium, and ammonium sulphate was added to a total concentration of 30 grams per liter of NH3. This solution was carefully clarified by known means to remove traces of magnesium, lead, nickel, molybdenum, copper and cobalt. The clarified solution was fed to the catholyte compartment of a diaphragm cell to maintain the concentration of the chromium at 15-35 grams per liter and the NHs at 40 grams per liter. Conditions of electrolysis were the same as in Example 1. The product obtained analyzed 98.8% chromium, 0.4% iron, 0.01% sulphur, 0.03% H20, and about .15% Oz.

The invention is not limited to the preferred embodiments but may be otherwise embodied or practiced within the scope of the following claims.

We claim:

1. The method of producing from chromite ores and the like, a solution suitable for electrodepositing chr0- mium, which comprises smelting the ore with carbon to form an alloy of iron, chromium and carbon and a slag containing most of the other ingredients of the ore, separating the alloy from the slag, casting the alloy into thin slabs which cool rapidly, dissolving the cast alloy by making it an anode in a solution of sulphuric acid and ammonium sulphate under conditions to form a solution containing ferrous sulphate, chromic and chromous sulphates, adding ferrochrome to adjust the pH of the solution to 1-3 and to reduce the iron to the ferrous state, heating said solution having a pH of 1-3 to a temperature sufficient to stabilize the chromium in the green chromic modification, then cooling said solution to crystallize a ferrous ammonium sulphate product, removing said ferrous ammonium sulphate product from the solution, crystallizing and removing ammonium chromic sulphate from its mother liquor, and redissolving the ammonium chromic sulphate.

, 2. The method of producing from chromite ores and the like, a solution suitable for electrodep ositing chromium, which comprises smelting the ore with carbon to form an alloy of iron, chromium and carbon and a slag containing most of the other ingredients of the ore, separating the alloy from the slag, casting the alloy into thin slabs which cool rapidly, dissolving the cast alloy by making .it an anode in a solution of sulphuric acid and ammonium sulphate and adding sulphur dioxide to maintain the iron in the ferrous state, withdrawing the anolyte from the cell, adding ferrochrome to the withdrawn anolyte to adjust the pH of the solution to 1-3, heating said solution having a pH of l-3 to a temperature sufiicient to stabilize the chromium in the green chromic modification, then cooling said solution to crystallize a ferrous ammonium sulphate product, removing said ferrous ammonium sulphate product from the solution, crystallizing and removing ammonium chromic sulphate from its mother liquor, and redissolving the ammonium chromic sulphate.

3. The method of producing electrolytic chromium from chromite ores and the like, which comprises smelting the ore with carbon to form an alloy of iron, chromium and carbon and a slag containing most of the other ingredients of the ore, separating the alloy from the slag, casting said alloy into thin shapes suitable for anodes which cool rapidly and making said shapes an anode in a two compartment electrolytic cell having an electrolyte of sulphuric acid and ammonium sulphate in the anode compartment and a solution of salt of a metal selected from the group consisting of iron and chromium in the catholyte compartment and an insoluble cathode, passing a unidirectional current through the cell to dissolve said alloy anode and deposit metal of said group on the cathode, withdrawing the anolyte from the cell, adding ferrochrome to the withdrawn anolyte to adjust the pH of the solution to 1-3 and to reduce the iron to the ferrous state, heating said solution having a pH of 1-3 to a temperature sufficient to stabilize the chromium in the green chromic modification, cooling said solution to crystallize aferrous ammonium sulphate product, removing said ferrous ammonium sulphate product from the solution, then crystallizing and removing ammonium chromic sulphate from the purified solution, redissolving the ammonium chromic sulphate and passing a unidirectional current through the am monium chromic sulphate solution to deposit chromium and regenerate a solution containing sulphuric acid and ammonium sulphate, and utilizing said regenerated solutionin the anode compartment of a cell for anodically dissolving a further quantity of said alloy.

References Cited in the file of this patent UNITED STATES PATENTS 526,114 Peacet et al. Sept. 18, 1894 1,343,725 Hultman June 15, 1920 1,477,965 Leaver Dec. 18, 1923 1,619,666 Gansen Mar, 1, 1927 1,502,035 Hasencl'ever July 22, 1934 2,374,454 Oliver Apr. 24, 1945 2,507,475 Lloyd May 9, 1950 2,507,476 Lloyd May 9, 1950 2,647,827 McGauley Aug. 4, 1953 FOREIGN PATENTS 477,381 Great Britain Dec. 23, 1937 224,065 Great Britain Nov. 6, 1924 437,497 Great Britain Oct. 30, 1935 419,365 Germany Sept. 28, 1925 OTHER REFERENCES Thompson: Transactions of the American Electrochemical Society, The Production of Chromates From Ferro-Chromium Anodes, vol. XLVI, 1924, pp. 51 to 65.

Stansfield: The Electric Furnace for Iron and Steel, McGraw Hill Co., 1923, pages -198.

Babor and Lehman: General College Chemistry, Crowell Co., New York, 1940, pages 343, 367. 

1. THE METHOD OF PRODUCING FROM CHROMITE ORES AND THE LIKE, A SOLUTION SUITABLE FOR ELECTRODEPOSITING CHROMIUM, WHICH COMPRISES SMELTING THE ORE WITH CARBON TO FORM AN ALLOY OF IRON, CHROMIUM AND CARBON AND A SLAG CONTAINING MOST OF THE OTHER INGREDIENTS OF THE ORE, SEPARATING THE ALLOY FROM THE SLAG, CASTING THE ALLOY INTO THIN SLABS WHICH COOL RAPIDLY, DISSOLVING THE CAST ALLOY BY MAKING IT AN ANODE IN A SOLUTION OF SULPHURIC ACID AND AMMONIUM SULPHATE UNDER CONDITIONS TO FORM A SOLUTION CONTAINING FERROUS SULPHATE, CHROMIC AND CHROMOUS SULPHATES, ADDING FERROCHROME TO ADJUST THE PH OF THE SOLUTION TO 1-3 AND TO REDUCE THE IRON TO THE FERROUS STATE, HEATING SAID SOLUTION HAVING A PH OF 1-3 TO A TEMPERATURE SUFFICIENT TO STABILIZE THE CHROMIUM IN THE GREEN CHROMIC MODIFICATION, THEN COOLING SAID SOLUTION TO CRYSTALLIZE A FERROUS AMMONIUM SULPHATE PRODUCT, REMOVING SAID FERROUS AMMONIUM SULPHATE PRODUCT FROM THE SOLUTION, CRYSTALLIZING AND REMOVING AMMONIUM CHROMIC SULPHATE FROM ITS MOTHER LIQUOR, AND REDISSOLVING THE AMMONIUM CHROMIC SULPHATE. 